This invention relates to a linear gain controlled amplifier.
In most communications systems, and particularly in wireless networks, the communication channels are subject to variable propagation losses. Thus there is often a need at the receiver and/or transmitter for some form of gain control to maintain the signal level.
The required accuracy and stability of the gain control elements vary depending on the system and its implementation. There is a general requirement, however, that the gain control be as accurate as possible to enhance system performance by reducing the effects of interference, improving battery life, etc.
Traditionally, transceivers use some form of closed loop to maintain the signal levels. The loop consists of a power detector, a loop filter, and a gain control element. The output of the power detector is compared with a reference voltage in a comparator, the output of which is filtered by the loop filter (low-pass filter) and applied to a control input of the gain control element. Unfortunately, most power detectors are either inaccurate or expensive and elaborate and may exhibit a control error of several decibels. The gain control elements themselves typically have non-linear control characteristics and suffer from poor repeatability and temperature variations which can necessitate extensive calibration.